The field of the invention is magnetic resonance imaging (MRI) systems and methods. More particularly, the invention relates to methods for free-breathing cardiac MR imaging using iterative image-based respiratory motion correction.
MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field, such as the so-called main magnetic field, B0, of an MRI system, the individual magnetic moments of the nuclei in the tissue attempt to align with this B0 field, but precess about it in random order at their characteristic Larmor frequency, Ω. If the substance, or tissue, is subjected to a so-called excitation electromagnetic field, B1, that is in the plane transverse to the B0 field and that has a frequency near the Larmor frequency, the net aligned magnetic moment, referred to as longitudinal magnetization, may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment, referred to as transverse magnetization. A signal is emitted by the excited nuclei or “spins,” after the excitation field, B1, is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed for spatial encoding. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences, which can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences, and also enable the development of new pulse sequences. The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence.
Because it requires time to acquire a complete k-space MR data set, subject motion presents a problem in many clinical applications. Motion due to respiration, cardiac motion, or peristalsis can produce image artifacts such as blurring or ghosting. For example, noninvasive evaluation of coronary artery disease (“CAD”) has been a major goal of coronary MRI. Due to the small diameter of the coronary arteries, a high spatial resolution coronary MRI is required for the accurate visualization of the arteries. However, this has been difficult to accomplish because the coronary arteries are in constant motion due to the cardiac and respiratory cycles.
There are many strategies used to suppress such artifacts caused by subject motion. These include cardiac or respiratory gating techniques that acquire MR data only during certain phases of the cardiac or respiratory cycle. For example, to correct for cardiac motion during coronary MRI, segments of k-space lines are acquired during a short diastolic rest period of the right coronary artery in each cardiac cycle. Thus, the subject is scanned while in a particular position, but the overall scan time is increased substantially because MR data is not acquired over substantial portions of each motion cycle.
Another technique for dealing with subject motion is to interleave so-called “navigator” pulse sequences into the scan to measure subject motion. Navigator pulse sequences may be used during a scan to periodically acquire subject motion information with which the acquired k-space MR image data may be retrospectively corrected. The interleaved navigator pulse sequences, however, can add considerable scan time and in some cases they can disrupt the magnetization equilibrium required by imaging pulse sequences.
As an example, a coronary MRI acquisition is typically performed during free-breathing with a respiratory motion compensation algorithm. A diaphragmatic navigator is used to measure the right hemi diaphragm (“RHD”) motion during the acquisition and to gate and correct for the respiratory motion of the heart. More specifically, before the acquisition of each k-space segment, the location of the RHD is monitored by the diaphragmatic navigator. If k-space segments are acquired when the RHD position is within a gating window timed around the respiratory end-expiration, the k-space segments are accepted for image reconstruction; otherwise, the k-space segments are rejected and reacquired in the next cardiac cycle. Typically, a five millimeter (“mm”) end expiratory gating window is used to gate data because increasing the window greatly reduces the accuracy of the factor used for correcting respiratory-induced heart motion. While this acceptance/rejection approach successfully suppresses the respiratory motion of the heart, it is hindered by low respiratory efficiency (defined as the percentage of k-space segments acquired within the gating window) that results from using such a narrow gating window and variability in the subject's breathing pattern. Low navigator efficacies, often around 30-50%, result in prolonged scan time and incomplete scans, making the acquisition of high-resolution cardiac images impractical.
Several methods have been proposed to increase the size of the gating window and thereby increase gating efficiency. For example, k-space weighting and phase ordering techniques, as well as a diminishing variance algorithm, have been shown to improve image quality over the conventional acceptance/rejection approach discussed above by using a larger gating window. However, the effectiveness of these techniques is based on the subject's breathing pattern; thus, changes in the subject's respiratory pattern can significantly impact the gating efficiency.
Self-gating navigators have also been proposed to estimate the respiratory motion of the heart directly from the acquired k-space lines rather than the RHD motion. However, these techniques only account for respiratory motion of the heart along the superior-inferior (“SI”) direction. The motion of the heart along anterior-posterior (“AP”) and right-left (“RL”) directions cannot be ignored for a gating window greater than seven mm and, therefore, must be accounted for in a motion compensation algorithm. Some three-dimensional navigators have been proposed to correct for the motion of the heart along the SI, AP, and RL directions. Also, rigid and affine transformations and non-rigid motion models have been used to estimate the respiratory motion of the heart before acquisition of k-space segments, and to correct the acquired k-space segments based on the estimated motion model. However, these algorithms involve either acquiring auxiliary pulses before the acquisition of k-space segments to generate a low resolution image and to estimate and correct for the heart respiratory motion, or modifying the k-space sampling scheme from Cartesian to radial to generate the low resolution image from the acquired inner k-space lines and to correct for the respiratory motion of the heart.
It would therefore be desirable to provide a method for acquiring high-resolution cardiac images and accurately compensating for respiratory motion. More specifically, it would be desirable to widen the gating window for acquiring data, thereby increasing navigator efficiency and shortening scan time, without respiratory-induced heart motion artifacts.